A preliminary application of a corona charge to articles of various metals and certain polymer plastics compositions is known to enhance wetting and bonding properties of their surface. Typically, the material or composition comprising an article has at least some dielectric properties and is in sheet, web or bulk form. For applying a corona charge, the article is fed in a controlled air gap between electrodes, one of which is energized with a high voltage electrical field and the other of which is grounded. The charge emitting electrode is typically comprised of a metal segment attached to a bar or beam.
During application, an air gap is maintained between the discharge electrode and the surface of the material on a support. The air gap typically has a lower dielectric breakdown voltage than does the material. As high voltage power is applied across the electrode, the air gap, the article and the article support, the air in the gap is believed to become ionized from acceleration of electrons to form a gaseous conductor comprising corona. The ionized air gap induces an electron avalanche which in turn creates oxidative molecules such as ozone. Oxidation of the recipient surface of the material affected by the corona increases surface energy which in turn enhances liquid wetting and adhesion promotion. While not completely understood, it is believed that the increased wetting ability is caused by small pores and crevices created by the corona in the contact area of the surface. Additionally, the easier flow tends to reduce stress concentration when the liquid solidifies.
By means of the foregoing, the corona treatment effects a surface increasingly receptive to printing inks, bonding, etc. It has been found particularly effective when used on polymeric materials, including the polyolefins, such as polypropylene, polyethylene and propylene-ethylene copolymers, and the polyesters marketed under the trademarks "Mylar", "Dacron", "Kodel", "Fortrel", etc. It is likewise effective with polyomides such as nylon, the fluorocarbons such as Teflon.TM., the vinyl polymers and copolymers such as polyvinylchloride, the polyvinyl ethers, etc., the polyurethanes, the cellulosics such as cellulose acetate, cellulose ethers, etc. When applied to the named compositions, utilization of corona treatment has been found particularly advantageous in products for the medical, automotive, printing and film industries.